Layer-by-layer deposition of thermoresponsive microgel thin films

We report investigations of the assembly of thermoresponsive, 4-acrylamidofluorescein-modified (indicated by an asterisk) poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc*) microgel thin films via a traditional alternate layer deposition protocol. In this work, pNIPAm-co-AAc* microgels have been synthesized and incorporated into thin films by alternatively exposing glass slides functionalized with 3-aminopropyltrimethoxysilane to pNIPAm-co-AAc* microgels, which act as polyanions, and poly(allylamine hydrochloride), which acts as a polycation. Using this method, pNIPAm-co-AAc* microgel thin films have been constructed as confirmed by fluorescence microscopy. Investigations were also conducted on the effect of the microgel deposition temperature on the film morphology and thermoresponsivity. In addition, the thermoresponsivities of the pNIPAm-co-AAc* microgel thin films were studied with respect to the microgel layer number, microgel deposition conditions, and pH. As expected from the solution behavior of the microgels, the films exhibited enhanced thermoresponsivity and hindered deswelling at pH values below and above the pK(a) of the microgel acrylic acid (AAc) groups, respectively. This result is expected due to charge repulsion of the AAc groups in the microgel network, which directly impacts the thermal deswelling of the hydrogel.